In 3GPP LTE system, each wireless frame has a length of 10 ms, and is divided into ten equally sized subframes. A downlink transmission time interval (TTI) is defined in one subframe. As shown in FIG. 1, each downlink subframe includes two slots. For a normal cyclic prefix (CP) length, each slot contains 7 OFDM symbols; for an extended cyclic-prefix length, each slot contains 6 OFDM symbols. In each subframe, the front n (n is equal to 1, 2 or 3) OFDM symbols are to transmit downlink control information including a physical downlink control channel (PDCCH) and other control information; the remaining OFDM symbols are to transmit Physical Downlink Shared Channel (PDSCH) or enhanced PDCCH (E PDCCH). The resource allocation granularity is a physical resource block (PRB). One PRB contains 12 consecutive subcarriers in frequency and corresponds to a time slot in time. In one subframe, two PRBs respectively located in two slots of the subframe while occupying same subcarriers are referred to as a PRB pair. In each PRB pair, each resource elements (RE) is the smallest unit of time-frequency resources, i.e., a subcarrier in frequency and an OFDM symbol in time. RE can be used for different functions, respectively. For example, some RE can be used to transmit cell-specific reference signal (CRS), user-specific demodulation reference signal (DMRS) and channel quality indicator reference signal (CSI-RS), etc.
In LTE system, multiple transmission modes are defined for transmitting data. For example, for downlink direction, there are closed loop multiple-input multiple-output (MIMO) transmission mode, open loop MIMO transmission mode, transmit diversity transmission, and so on. For one transmission mode, the system configures a normal downlink control informal (DCI) format which is to complete normal data transmission under this kind of transmission mode. Meanwhile, the base station also configures a User Equipment (UE) to detect one kind of fallback DCI format. The fallback DCI format usually has fewer bits, adopts a more conservative way to schedule data, such as transmit diversity or single-antenna sending data, and thus has high reliability.
In LTE system, DCIs which are sent to different UEs or have different functions can be independently coded and transmitted. When performing physical resource mapping on PDCCH, taking control channel Element (CCE) as a unit; when performing physical resource mapping on EPDCCH, taking enhanced CCE (ECCE) as a unit. In following description, when it is not needed to specifically distinguish PDCCH and EPDCCH, they can be collectively referred to as (E)PDCCH; accordingly, CCE and ECCE can be collectively referred to as (E)CCE. Specifically, modulation symbols of one (E)PDCCH can be mapped to L (E)CCE, where L can be equal to 1, 2, 4, 16 or 32, and L can also be known as aggregation level of (E)PDCCH. (E)PDCCH fixed adopts QPSK modulation method; according to bit number of control information and link condition of UE, the base station can select aggregation level of (E)CCE for sending (E)PDCCH.
In the existing LTE versions, downlink data transmission based on QPSK, 16 Quadrature Amplitude Modulation (QAM) and 64 QAM can be supported. Table 1 shows indexes of modulation coding scheme (MCS) and transport block size (TBS) which are used for downlink transmission. Specifically, in the existing LTE versions, in DCI information, 5 bits are used to indicate MCS and TBS information, in which 29 code words simultaneously indicate modulation mode and TBS, the last 3 code words only indicate modulation mode, while TBS information can be obtained according to previous DCI information and be used for retransmission of PDSCH.
TABLE 1MCS and TBS for PDSCH transmissionMCS indexmodulationIMCSorder QmTBS index ITBS0201212223234245256267278289291049114101241113412144131541416415176151861619617206182161922620236212462225623266242762528626292reserved304316
Accordingly, in order to support the base station scheduling downlink PRB resource, UE needs to report channel status indication (CSI) information including channel quality indicator (CQI) information. Table 2 shows modulation mode and code rate, etc. of each CQI index. Specifically, in the existing LTE version, 4 bits are used to report CQI information. Consistent with MCS configuration in the existing LTE version, in CQI measurement, only situations in which downlink data transmissions based on QPSK, 16 QAM and 64 QAM can be currently supported.
TABLE 2CQI informationCQIcode rate ×indexcoding1024efficiency0invalid value1QPSK780.15232QPSK1200.23443QPSK1930.37704QPSK3080.60165QPSK4490.87706QPSK6021.1758716 QAM3781.4766816 QAM4901.9141916 QAM6162.40631064 QAM4662.73051164 QAM5673.32231264 QAM6663.90231364 QAM7724.52341464 QAM8735.11521564 QAM9485.5547
In LTE version 12, in order to add a peak downlink transmission rate of a small cell, one possible candidate technique is to support PDSCH transmission based on 256 QAM modulation. In a typical network configuration, for example, uses a macro base station on lower frequencies to achieve large coverage; and sets some small base stations on higher frequencies to achieve hotspot coverage. Since the small base station uses high frequency point, its propagation characteristics determine inter-cell interference is small and there is not interference from the macro base station, thus, signal to interference and noise ratio of UE in smaller cell can be very high and can sufficiently support downlink transmission based on 256 QAM. In order to introduce the support for 256 QAM, it is needed to modify processing way of MCS and CQI in the existing LTE specifications and solve a series of resulting problems.